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But suppose that we identify alteration with Motion on the
ground that Motion itself results in difference: how then do we
proceed to define Motion?
It may roughly be characterized as the passage from the
potentiality to its realization. That is potential which can
either pass into a Form- for example, the potential statue- or
else pass into actuality- such as the ability to walk: whenever
progress is made towards the statue, this progress is Motion; and
when the ability to walk is actualized in walking, this walking
is itself Motion: dancing is, similarly, the motion produced by
the potential dancer taking his steps.
In the one type of Motion a new Form comes into existence created
by the motion; the other constitutes, as it were, the pure Form
of the potentiality, and leaves nothing behind it when once the
motion has ceased. Accordingly, the view would not be
unreasonable which, taking some Forms to be active, others
inactive, regarded Motion as a dynamic Form in opposition to the
other Forms which are static, and further as the cause of
whatever new Form ensues upon it. To proceed to identify this
bodily motion with life would however be unwarrantable; it must
be considered as identical only in name with the motions of
Intellect and Soul.
That Motion is a genus we may be all the more confident in virtue
of the difficulty- the impossibility even- of confining it within
a definition.
But how can it be a Form in cases where the motion leads to
deterioration, or is purely passive? Motion, we may suggest, is
like the heat of the sun causing some things to grow and
withering others. In so far as Motion is a common property, it is
identical in both conditions; its apparent difference is due to
the objects moved.
Is, then, becoming ill identical with becoming well? As motions
they are identical. In what respect, then, do they differ? In
their substrates? or is there some other criterion?
This question may however be postponed until we come to consider
alteration: at present we have to discover what is the constant
element in every motion, for only on this basis can we establish
the claim of Motion to be a genus.
Perhaps the one term covers many meanings; its claim to generic
status would then correspond to that of Being.
As a solution of the problem we may suggest that motions
conducing to the natural state or functioning in natural
conditions should perhaps, as we have already asserted, be
regarded as being in a sense Forms, while those whose direction
is contrary to nature must be supposed to be assimilated to the
results towards which they lead.
But what is the constant element in alteration, in growth and
birth and their opposites, in local change? What is that which
makes them all motions? Surely it is the fact that in every case
the object is never in the same state before and after the
motion, that it cannot remain still and in complete inactivity
but, so long as the motion is present, is continually urged to
take a new condition, never acquiescing in Identity but always
courting Difference; deprived of Difference, Motion perishes.
Thus, Difference may be predicated of Motion, not merely in the
sense that it arises and persists in a difference of conditions,
but in the sense of being itself perpetual difference. It follows
that Time, as being created by Motion, also entails perpetual
difference: Time is the measure of unceasing Motion, accompanying
its course and, as it were, carried along its stream.
In short, the common basis of all Motion is the existence of a
progression and an urge from potentiality and the potential to
actuality and the actual: everything which has any kind of motion
whatsoever derives this motion from a pre-existent potentiality
within itself of activity or passivity.
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